Two more men were arrested on Wednesday in the Welsh city of Newport over the recent explosion at a subway station in west London, bringing total held to five, media reports said.
A 48-year-old man and a 30-year-old man were detained under the Terrorism Act after a search at an address in Newport in south Wales.
The two were arrested in connection with the Friday explosion at a packed, rush-hour carriage at Parsons Green subway station, which left 30 people injured.
The Terrorism Act allows police to detain suspects without charge beyond the four days allowed for suspects connected to other crimes.
A third suspect, a 25-year-old, who is reported to be a painter-decorator, was held in a counter-terrorism raid in Newport, South Wales, on Tuesday.
Two other suspects -- Yahyah Farroukh, 21, and an 18-year-old man -- remain in custody after detectives were granted more time to question them over the bomb attack, caused by an improvised explosive device, which failed to fully detonate on the London underground train.
Scotland Yard said magistrates had granted warrants allowing the 18-year-old to be held until Saturday, and Farroukh until Thursday.
The news came as fresh CCTV footage emerged that appears to show the Parsons Green bomb suspect is on his way to plant his home-made device.
The Islamic State has claimed responsibility for the fifth attack in the country over the past six months.
Previous attacks in London this year at Westminster Bridge, London Bridge and Finsbury Park as well as a blast at an Ariana Grande concert in Manchester killed dozens of people and injured more than 150.